The present invention generally relates to a coupling structure, a resonator excitation structure and a filter mainly used for microwave or millimeter-wave band coplanar-waveguide circuits.
In the prior art, two kinds of couplings are known as a resonator excitation structure at input/output of coplanar-waveguide circuits such as filters. One is capacitive coupling where an open end of an exciting-line is close to a resonator. The other is inductive coupling where an exciting line is directly connected to a resonator.
FIG. 1 is a plan view of an excitation structure employing a conventional capacitive coupling (See Non-patent Document #1). A coplanar-waveguide circuit 1 includes an exciting line 4 longitudinally running at the center thereof. An end of the exciting line 4 is extended laterally like a T-shape. The T-shape portion of the exciting line 4 faces a T-shape portion of a resonator 6 via a gap to form an excitation portion 5. The sides of the coplanar plane circuit 1 are covered with corresponding ground conductors 2, 3.
FIG. 2 is a plan view of an excitation structure employing a conventional inductive coupling (See Non-patent Document #2). An exciting line 4 is directly connected to a short-circuit portion between an end of a resonator 6 and a ground conductor 3 to form an excitation portion 5.
FIG. 3 is a plan view of an excitation structure employing a conventional inductive coupling (See Non-patent Document #3). An exciting line 4 is directly connected to an end of a resonator 6, and a cross-shape line is connected to ground plates 2, 3 at its corresponding ends to form an excitation portion 5.
[Non-patent Document #1] “A 5 GHz Band Coplanar-Waveguide High Temperature superconducting Filter Employing T-shaped Input/Output Coupling Structure and Quarter-Wavelength Resonator” by Koizumi, Sato, Narahashi, Technical Report of IEICE, MW2004-25, pp. 55-60, May. 2004.
[Non-patent Document #2] “Design of a 5 GHz Bandpass Filter Using CPW Quarter-Wavelength Spiral Resonators” by Kawaguchi, Ma, Kobayashi, Proceedings of the 2004 IEICE Society Conference, C-2-81, November 2004.
[Non-patent Document #3] “Design of a 5 GHz Interdigital Bandpass Filter Using CPW Quarter-Wavelength Resonators” by Kawaguchi, Ma, Kobayashi, Proceedings of the 2004 IEICE Society Conference, C-2-80, November 2004.
The above mentioned conventional excitation structures shown in FIGS. 1˜3 have problems discussed below.
In the resonator excitation structure using capacitive coupling as shown in FIG. 1, its external coupling is in general weaker than that in a resonator excitation structure using inductive coupling. When designing bandpass filters using capacitive coupling, in order to obtain a desired external coupling strength, the open end portion of the exciting line must be placed near a portion of the resonator where charges are concentrated. However, if such a charge concentrated portion is not at an outer area, the length of the exciting line must be long enough to ensure a sufficient external coupling strength. That enlarges the excitation structure area of the planar circuit substrate, adversely affects a next stage resonator, and degrades entire circuit characteristics, which are problems.
On the other hand, in a resonator excitation structure using direct connected inductive couplings as shown in FIG. 2 or 3, its external coupling is too strong. Accordingly an exciting line must be directed coupled to the resonator near a short-circuit portion in case of quarter-wavelength resonators, and it is difficult to place the exciting line near the center of plane circuit substrate. When a housing can be considered to be a cut-off waveguide, undesired transmission modes or propagation modes are strongly excited and the circuit characteristics are degraded.
In addition, when adjusting the external coupling strength after manufacturing a planar circuit substrate and circuit pattern, such adjustment also affects the resonant frequency of the resonator. Therefore, it is impossible to independently adjust the external coupling parameter only. As an example explaining this problem, FIG. 4 shows a resonator excitation structure in which an exciting line is directly connected to quarter-wavelength spiral resonator to form inductive coupling. By removing an adjusting portion 7 (indicated by hatched lines) of a ground conductor 2 after manufacturing a circuit pattern, it is possible to increase a gap width g between the ground conductor 2 and a resonator 6 and increase its external Q or weaken external coupling strength. FIG. 5 is a graph showing that the external Q and the resonant frequency of the resonator 6 vary with respect to the gap width g. As clearly shown in FIG. 5, the increase of the gap width g increases not only the external Q but also the resonant frequency of the resonator 6.
Although the above explanation is given about the excitation structure of resonators, these problems may occur at a connecting portion between any circuit portions and signal input/output lines in planar circuits.